lxd/instance/drivers/driver/qemu: Switch to threads locking mode and writeback cache mode for BTRFS
lxd/instance/drivers/driver/qemu: Switch to threads locking mode and writeback cache mode for BTRFS

When taking snapshots of running VMs on BTRFS backed filesystems this was causing issues when doing BTRFS volume send/recv.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd